Rob Sewell shows how to iterate through a collection of Pester tests:
The problem with Test Cases is that we can only easily loop through one collection, but as Pester is just PowerShell we can simply use ForEach if we wanted to loop through multiple ones, like instances and then databases.
I like to use the ForEach method as it is slightly quicker than other methods. It will only work with PowerShell version 4 and above. Below that version you need to pipe the collection to For-EachObject.
Check it out for a good explanation of running groups of Powershell tests.